Material supply apparatus for extreme ultraviolet light source having a filter constructed with a plurality of openings fluidly coupled to a plurality of through holes to remove non-target particles from the supply material

ABSTRACT

A filter is used in a target material supply apparatus and includes a sheet having a first flat surface and a second opposing flat surface, and a plurality of through holes. The first flat surface is in fluid communication with a reservoir that holds a target mixture that includes a target material and non-target particles. The through holes extend from the second flat surface and are fluidly coupled at the second flat surface to an orifice of a nozzle. The sheet has a surface area that is exposed to the target mixture, the exposed surface area being at least a factor of one hundred less than an exposed surface area of a sintered filter having an equivalent transverse extent to that of the sheet.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.13/112,784, filed on May 20, 2011 and titled FILTER FOR MATERIAL SUPPLYAPPARATUS OF AN EXTREME ULTRAVIOLET LIGHT SOURCE, which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The disclosed subject matter relates to a filter for use in a targetmaterial supply apparatus.

BACKGROUND

Extreme ultraviolet (“EUV”) light, for example, electromagneticradiation having wavelengths of around 50 nm or less (also sometimesreferred to as soft x-rays), and including light at a wavelength ofabout 13 nm, can be used in photolithography processes to produceextremely small features in substrates, for example, silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material into a plasma state that has an element, forexample, xenon, lithium, or tin, with an emission line in the EUV range.In one such method, often termed laser produced plasma (“LPP”), therequired plasma can be produced by irradiating a target material, forexample, in the form of a droplet, stream, or cluster of material, withan amplified light beam that can be referred to as a drive laser. Forthis process, the plasma is typically produced in a sealed vessel, forexample, a vacuum chamber, and monitored using various types ofmetrology equipment.

SUMMARY

In some general aspects, an apparatus supplies a target material to atarget location. The apparatus includes a reservoir that holds a targetmixture that includes the target material and non-target particles; afirst filter through which the target mixture is passed; a second filterthrough which the target mixture is passed; and a supply system thatreceives the target mixture that has passed through the first and secondfilters and that supplies the target mixture to the target location. Thesecond filter includes a set of through holes having uniformly-sizedcross-sectional widths and has a surface area that is exposed to thetarget mixture, the exposed surface area being at least a factor of onehundred less than an exposed surface area of a sintered filter having anequivalent transverse extent to that of the second filter.

Implementations can include one or more of the following features. Forexample, the second filter can receive the target mixture that haspassed through the first filter. Or, the first filter can receive thetarget mixture that has passed through the second filter.

The second filter can include another set of uniformly-sized throughholes having a transverse size that is different from a transverse sizeof the uniformly-sized through holes of the set.

The first filter can be selected from the group of sintered filters andmesh filters.

The first filter can also include a set of through holes havinguniformly-sized cross-sectional widths and can have a surface area thatis exposed to the target mixture, the exposed surface area being atleast a factor of one hundred less than an exposed surface area of asintered filter having an equivalent transverse extent to that of thesecond filter. The cross-sectional widths of the through holes of thefirst filter set can be different from the cross-sectional widths of thethrough holes of the second filter set.

The second filter can have a thickness along a longitudinal directionthat is large enough to withstand a pressure differential across thesecond filter.

Each hole in the set of through holes of the second filter can have across-sectional width that is less than 10 μm. The cross-sectional widthof each hole in the set of through holes of the second filter can varyno more than 20% from the cross-sectional width of each of the otherholes in the set of through holes of the second filter.

The supply system can include a nozzle that defines an orifice throughwhich the target mixture is passed. The nozzle can direct the targetmixture toward the target location through the orifice. Thecross-sectional width of each hole in the set of through holes of thesecond filter can be less than a cross-sectional width of the nozzleorifice. The supply system can be configured to generate droplets of thetarget material.

At least one of the first and second filters can be made at least inpart of tungsten, titanium, molybdenum, nickel, tantalum, or othermetal, quartz, glass, or ceramic material.

The through holes of the set in the second filter can be sized to blockat least some of the non-target particles.

The target mixture can be a fluid. The fluid can be a liquid, a gas, or,to some extent, a plastic solid.

The second filter can be made of glass or tungsten. The second filtercan be a non-mesh and a non-sintered filter.

The second filter can include etched holes or collimated capillaryholes. The first filter can be a sintered filter. The second filter caninclude micro-machined holes.

The first filter can include holes that are sized to block at least someof the non-target particles. The second filter can include holes thatare sized to block at least some of the non-target particles.

The second filter can be made of a material that is different from amaterial of the first filter. The second filter can be made of glass.

The target material can be pure tin. In this case, the first filter canbe made of a material that is not perfectly compatible with tin, forexample, the material of the first filter may be readily corroded oreroded by liquid tin. For example, the first filter can be made oftitanium, stainless steel, or a material that is formable, sinterable,ductile enough to withstand mounting after sintering. In this case, thesecond filter is more compatible with tin. For example, the secondfilter can be made of glass, tungsten, nickel, other refractory metal,quartz, or a suitable ceramic material (such as alumina, siliconcarbide, silicon nitride, TiN, etc.).

In other general aspects, an apparatus includes a reservoir that holds atarget mixture that includes a target material and non-target particles;a first filter through which the target mixture is passed, the firstfilter being made of a first material; a second filter through which thetarget mixture is passed, the second filter being made of a secondmaterial that is different from the first material; a supply system thatreceives the target mixture that has passed through the first and secondfilters and that supplies the target mixture to the target location; aradiation source that supplies radiation to the target location tothereby strike the target mixture; and a collection system that capturesand directs extreme ultraviolet light generated by the target mixturestruck by the radiation.

Implementations can include one or more of the following features. Forexample, the second filter can receive the target mixture that haspassed through the first filter.

The target material can include tin and the second material can includeglass or tungsten.

The second filter can include a set of through holes havinguniformly-sized cross-sectional widths.

In another general aspect, a filter that is used in a target materialsupply apparatus includes a plurality of through holes that are fluidlycoupled at a first end to a reservoir that holds a target mixture thatincludes a target material and non-target particles, and are fluidlycoupled at a second end to an orifice of a nozzle. A cross-sectionalwidth of each through hole of the plurality of through holes of thefilter varies no more than 20% from the cross-sectional width of each ofthe other through holes of the plurality of through holes and thecross-sectional width of each hole of the plurality of through holes isless than a height of the hole.

Implementations can include one or more of the following features. Forexample, the through holes can be cylindrically-shaped. The number andthe cross-sectional width of the holes of the plurality can be chosen sothat a pressure drop across the filter is negligible after the targetmixture fills a volume between the filter and the nozzle and the targetmixture flows through the nozzle orifice. The cross-sectional widths ofthe holes can be sized to block at least some of the non-targetparticles.

The filter can be made of a material that is compatible with the targetmaterial. The filter can be made of tungsten or glass or other ceramicif the target material includes tin.

Each of the holes can have a circular cross section and thecross-sectional width of the hole can be a diameter of its circularcross section.

The filter can also include a plurality of openings, each opening beingbetween the first end of the reservoir and a set of holes such that theopening fluidly couples the first end of the reservoir to the set ofholes. Each opening can have a cross-sectional width; and each hole inthe set of holes can have a cross-sectional width that is smaller thanthe cross-sectional width of the opening to which the hole is fluidlycoupled. Each of the openings can have a circular cross section and thecross-sectional width of the opening is a diameter of its circular crosssection. The cross-sectional widths of the holes can be sized to blockat least some of the non-target particles.

In another general aspect, a filter for use in a target material supplyapparatus includes a plurality of through holes that are fluidly coupledat a first end to a reservoir that holds a target mixture that includesa target material and non-target particles, and are fluidly coupled at asecond end to an orifice of a nozzle; and a plurality of openings, eachopening being between the first end of the reservoir and a group ofthrough holes such that the opening fluidly couples the first end of thereservoir to the group of through holes. Each opening has across-sectional width, and each through hole in the group of throughholes has a cross-sectional width that is smaller than thecross-sectional width of the opening to which the through hole isfluidly coupled.

Implementations can include one or more of the following features. Forexample, the through holes can be cylindrical and have a uniformcross-sectional width along an axial length of the through hole. Thethrough holes can be formed as a capillary array.

The plurality of openings can be defined between a first surface thatfaces the reservoir and the set of through holes. The plurality ofthrough holes can be defined between a second surface that faces thenozzle and the plurality of openings.

In other general aspects, a device for generating light includes aradiation source that supplies radiation to a target location; a supplysystem that supplies a target mixture to the target location such that aplasma is formed when a target material within the target mixture isirradiated by the supplied radiation; and a filter configured to removeat least some non-target particles from the target mixture before thetarget mixture reaches the target location. The filter includes aplurality of through holes that are fluidly coupled at a first end to areservoir that holds the target mixture that includes the targetmaterial and the non-target particles, and are fluidly coupled at asecond end to the supply system; and a plurality of openings, eachopening being between the first end of the reservoir and a group ofthrough holes such that the opening fluidly couples the first end of thereservoir to the group of through holes.

Implementations can include one or more of the following features. Forexample, each opening can have a cross-sectional width, and each throughhole in the group of through holes can have a cross-sectional width thatis smaller than the cross-sectional width of the opening to which thethrough hole is fluidly coupled.

In another general aspect, a filter for use in a target material supplyapparatus includes a sheet having a first flat surface and a secondopposing flat surface, the first flat surface being in fluidcommunication with a reservoir that holds a target mixture that includesa target material and non-target particles; and a plurality of throughholes extending from the second flat surface and being fluidly coupledat the second flat surface to an orifice of a nozzle. The sheet has asurface area that is exposed to the target mixture, the exposed surfacearea being at least a factor of one hundred less than an exposed surfacearea of a sintered filter having an equivalent transverse extent to thatof the sheet.

Implementations can include one or more of the following features. Forexample, the exposed surface area can be at least a factor of tenthousand less than an exposed surface area of a sintered filter havingan equivalent transverse extent to that of the sheet. The filter canalso include a plurality of openings extending from the first flatsurface, each opening fluidly coupling a group of through holes to thereservoir.

In other general aspects, a method of filtering includes holding atarget mixture that includes a target material and non-target particlesin a reservoir; removing, using a first filter, at least some of thenon-target particles of the target mixture; removing, using a secondfilter that has a set of through holes having uniformly-sizedcross-sectional widths, at least some of the non-target particles of thetarget mixture; controlling, using a supply system, a flow of the targetmixture that passed through the second filter; and directing the targetmixture passed through the second filter to a target location thatreceives an amplified light beam to thereby convert the target materialof the target mixture into a plasma state.

Implementations can include one or more of the following features. Forexample, removing at least some of the non-target particles of thetarget mixture using the second filter can include removing at leastsome of the non-target particles of the target mixture that remain afterhaving passed through the first filter.

The method can also include removing, using the second filter, at leastsome non-target particles of the target mixture that were introducedinto the target mixture by the first filter.

The target mixture flow can be controlled by passing the target mixturefrom the second filter through an orifice of the supply system.

DRAWING DESCRIPTION

FIG. 1 is a block diagram of a laser produced plasma (LPP) extremeultraviolet (EUV) light source;

FIGS. 2-4 are schematic cross-sectional diagrams of exemplary targetmaterial supply apparatuses of the light source of FIG. 1;

FIG. 5 is a schematic cross-sectional diagram showing an exemplaryfilter design within the target material supply apparatuses of FIGS.2-4;

FIG. 6A is a schematic cross-sectional diagram showing an exemplaryfilter that can be used in the design of FIG. 5;

FIG. 6B is a schematic cross-sectional diagram of a magnified portion ofthe filter of FIG. 6A that shows details of holes formed therein;

FIG. 7 is a schematic cross-sectional diagram of a bulk substance thatis used to form the filter in FIGS. 6A and 6B;

FIG. 8A is a top view of the bulk substance of FIG. 7 showing openingsformed therein;

FIG. 8B is a cross-sectional view taken along 8B-8B of the bulksubstance of FIG. 8A;

FIGS. 9-12, 13A, and 14A are cross-sectional views showing each step inthe process of forming the filter of FIGS. 6A and 6B from the bulksubstance of FIG. 7;

FIG. 13B is a schematic cross-sectional diagram of a magnified portionof the bulk substance of FIG. 13A;

FIG. 14B is a schematic cross-sectional diagram of a magnified portionof the bulk substance of FIG. 14A;

FIG. 15 is a schematic cross-sectional diagram showing an exemplaryfilter design within the target material supply apparatuses of FIGS.2-4;

FIG. 16 is a schematic cross-sectional diagram of a bulk substance thatis used to form the filter in FIG. 15;

FIG. 17 is a schematic cross-sectional diagram of the filter in FIG. 15;

FIG. 18A is a perspective view of an exemplary non-mesh, non-sinteredfilter that can be used in the target material supply apparatuses ofFIGS. 2-4; and

FIG. 18B is a perspective view of an exemplary sintered filter havingthe same transverse extent of the exemplary filter of FIG. 18A.

DESCRIPTION

Referring to FIG. 1, an LPP EUV light source 100 is formed byirradiating a target mixture 114 at a target location 105 with anamplified light beam 110 that travels along a beam path toward thetarget mixture 114. The target location 105, which is also referred toas the irradiation site, is within an interior 107 of a vacuum chamber130. When the amplified light beam 110 strikes the target mixture 114, atarget material within the target mixture 114 is converted into a plasmastate that has an element with an emission line in the EUV range. Thecreated plasma has certain characteristics that depend on thecomposition of the target material within the target mixture 114. Thesecharacteristics can include the wavelength of the EUV light produced bythe plasma and the type and amount of debris released from the plasma.

The light source 100 also includes a target material delivery system 125that delivers, controls, and directs the target mixture 114 in the formof liquid droplets, a liquid stream, solid particles or clusters, solidparticles contained within liquid droplets or solid particles containedwithin a liquid stream. The target mixture 114 includes the targetmaterial such as, for example, water, tin, lithium, xenon, or anymaterial that, when converted to a plasma state, has an emission line inthe EUV range. For example, the element tin can be used as pure tin(Sn); as a tin compound, for example, SnBr₄, SnBr₂, SnH₄; as a tinalloy, for example, tin-gallium alloys, tin-indium alloys,tin-indium-gallium alloys, or any combination of these alloys. Thetarget mixture 114 can also include impurities such as non-targetparticles. Thus, in the situation in which there are no impurities, thetarget mixture 114 is made up of only the target material. The targetmixture 114 is delivered by the target material delivery system 125 intothe interior 107 of the chamber 130 and to the target location 105.

This description relates to the use of a filter and a method offiltering within the target material delivery system 125 for removingthe impurities (such as non-target particles) within the target mixture114. A description of the components of the light source 100 willinitially be described as background before a detailed description ofthe target material delivery system 125.

The light source 100 includes a drive laser system 115 that produces theamplified light beam 110 due to a population inversion within the gainmedium or mediums of the laser system 115. The light source 100 includesa beam delivery system between the laser system 115 and the targetlocation 105, the beam delivery system including a beam transport system120 and a focus assembly 122. The beam transport system 120 receives theamplified light beam 110 from the laser system 115, and steers andmodifies the amplified light beam 110 as needed and outputs theamplified light beam 110 to the focus assembly 122. The focus assembly122 receives the amplified light beam 110 and focuses the beam 110 tothe target location 105.

In some implementations, the laser system 115 can include one or moreoptical amplifiers, lasers, and/or lamps for providing one or more mainpulses and, in some cases, one or more pre-pulses. Each opticalamplifier includes a gain medium capable of optically amplifying thedesired wavelength at a high gain, an excitation source, and internaloptics. The optical amplifier may or may not have laser mirrors or otherfeedback devices that form a laser cavity. Thus, the laser system 115produces an amplified light beam 110 due to the population inversion inthe gain media of the laser amplifiers even if there is no laser cavity.Moreover, the laser system 115 can produce an amplified light beam 110that is a coherent laser beam if there is a laser cavity to provideenough feedback to the laser system 115. The term “amplified light beam”encompasses one or more of: light from the laser system 115 that ismerely amplified but not necessarily a coherent laser oscillation andlight from the laser system 115 that is amplified and is also a coherentlaser oscillation.

The optical amplifiers in the laser system 115 can include as a gainmedium a filling gas that includes CO₂ and can amplify light at awavelength of between about 9100 and about 11000 nm, and in particular,at about 10600 nm, at a gain greater than or equal to 1000. Suitableamplifiers and lasers for use in the laser system 115 can include apulsed laser device, for example, a pulsed, gas-discharge CO₂ laserdevice producing radiation at about 9300 nm or about 10600 nm, forexample, with DC or RF excitation, operating at relatively high power,for example, 10 kW or higher and high pulse repetition rate, forexample, 50 kHz or more. The optical amplifiers in the laser system 115can also include a cooling system such as water that can be used whenoperating the laser system 115 at higher powers.

The light source 100 includes a collector mirror 135 having an aperture140 to allow the amplified light beam 110 to pass through and reach thetarget location 105. The collector mirror 135 can be, for example, anellipsoidal mirror that has a primary focus at the target location 105and a secondary focus at an intermediate location 145 (also called anintermediate focus) where the EUV light can be output from the lightsource 100 and can be input to, for example, an integrated circuitlithography tool (not shown). The light source 100 can also include anopen-ended, hollow conical shroud 150 (for example, a gas cone) thattapers toward the target location 105 from the collector mirror 135 toreduce the amount of plasma-generated debris that enters the focusassembly 122 and/or the beam transport system 120 while allowing theamplified light beam 110 to reach the target location 105. For thispurpose, a gas flow can be provided in the shroud that is directedtoward the target location 105.

The light source 100 can also include a master controller 155 that isconnected to a droplet position detection feedback system 156, a lasercontrol system 157, and a beam control system 158. The light source 100can include one or more target or droplet imagers 160 that provide anoutput indicative of the position of a droplet, for example, relative tothe target location 105 and provide this output to the droplet positiondetection feedback system 156, which can, for example, compute a dropletposition and trajectory from which a droplet position error can becomputed either on a droplet by droplet basis or on average. The dropletposition detection feedback system 156 thus provides the dropletposition error as an input to the master controller 155. The mastercontroller 155 can therefore provide a laser position, direction, andtiming correction signal, for example, to the laser control system 157that can be used, for example, to control the laser timing circuitand/or to the beam control system 158 to control an amplified light beamposition and shaping of the beam transport system 120 to change thelocation and/or focal power of the beam focal spot within the chamber130.

The target material delivery system 125 includes a target materialdelivery control system 126 that is operable in response to a signalfrom the master controller 155, for example, to modify the release pointof the droplets as released by a target material supply apparatus 127 tocorrect for errors in the droplets arriving at the desired targetlocation 105.

Additionally, the light source 100 can include a light source detector165 that measures one or more EUV light parameters, including but notlimited to, pulse energy, energy distribution as a function ofwavelength, energy within a particular band of wavelengths, energyoutside of a particular band of wavelengths, and angular distribution ofEUV intensity and/or average power. The light source detector 165generates a feedback signal for use by the master controller 155. Thefeedback signal can be, for example, indicative of the errors inparameters such as the timing and focus of the laser pulses to properlyintercept the droplets in the right place and time for effective andefficient EUV light production.

The light source 100 can also include a guide laser 175 that can be usedto align various sections of the light source 100 or to assist insteering the amplified light beam 110 to the target location 105. Inconnection with the guide laser 175, the light source 100 includes ametrology system 124 that is placed within the focus assembly 122 tosample a portion of light from the guide laser 175 and the amplifiedlight beam 110. In other implementations, the metrology system 124 isplaced within the beam transport system 120. The metrology system 124can include an optical element that samples or re-directs a subset ofthe light, such optical element being made out of any material that canwithstand the powers of the guide laser beam and the amplified lightbeam 110. A beam analysis system is formed from the metrology system 124and the master controller 155 since the master controller 155 analyzesthe sampled light from the guide laser 175 and uses this information toadjust components within the focus assembly 122 through the beam controlsystem 158.

Thus, in summary, the light source 100 produces an amplified light beam110 that is directed along the beam path to irradiate the target mixture114 at the target location 105 to convert the target material within themixture 114 into plasma that emits light in the EUV range. The amplifiedlight beam 110 operates at a particular wavelength (that is alsoreferred to as a source wavelength) that is determined based on thedesign and properties of the laser system 115. Additionally, theamplified light beam 110 can be a laser beam when the target materialprovides enough feedback back into the laser system 115 to producecoherent laser light or if the drive laser system 115 includes suitableoptical feedback to form a laser cavity.

Referring to FIG. 2, in an exemplary implementation, a target materialsupply apparatus 227 includes two chambers, a first chamber 200 (whichis also referred to as a bulk material chamber) and a second chamber 205(which is also referred to as a reservoir) fluidly coupled to the firstchamber 200 by a pipe 210 that can be fitted with a valve to control theflow of material between the first chamber 200 and the second chamber205. The first and second chambers 200, 205 may be hermetically sealedvolumes with independent, active pressure controllers 202, 207. Thefirst and second chambers 200, 205, and the pipe 210 can be thermallycoupled to one or more heaters that control the temperature of the firstand second chambers 200, 205 and the pipe 210. Additionally, theapparatus 227 can also include one or more level sensors 215, 220 thatdetect an amount of substance within each of the respective chambers200, 205. The output of the level sensors 215, 220 can be fed to thecontrol system 126, which is also connected to the pressure controllers202, 207.

In operation, an operator fills the first chamber 200 with a bulksubstance 225, and heats up the substance 225 using the heater thermallycoupled to the first chamber 200 until the bulk substance 225 becomes afluid, which can be a liquid, a gas, or a plasma. The resultant fluidcan be referred to as a target mixture 230 that includes the targetmaterial plus other non-target particles. The non-target particles areimpurities in the target mixture 230 that are removed by one or morefilters (such as first and second filters 235, 240) in the apparatus227. The pipe 210 and the second chamber 205 may also be heated by theirrespective heaters to maintain the target mixture 230 as a fluid.

The apparatus 227 also includes a supply system 245 at the output of thesecond chamber 205, following the second filter 240. The supply system245 receives the target mixture 230 that has passed through the firstand second filters 235, 240 and supplies the target mixture in the formof droplets 214 to the target location l05. To this end, the supplysystem 245 can include a nozzle 250 defining an orifice 255 throughwhich the target mixture 230 escapes to form the droplets 214 of thetarget mixture. The output of the droplets 214 can be controlled by anactuator such as a piezoelectric actuator. Additionally, the supplysystem 245 can include other regulating or directing components 260downstream of the nozzle 250. The nozzle 250 and/or the directingcomponents 260 direct the droplets 214 (which is the target mixture 230that has been filtered to include the target material and a lot less ofthe impurities) to the target location 105.

The control system 126 receives inputs from the level sensors 215, 220,and controls the heaters to melt a given amount of the substance 225.The control system 126 also controls the pressure in each of thechambers 200, 205 and the opening and closing of the valve in the pipe210. A description of an exemplary arrangement of the first and secondchambers 200, 205 is found in U.S. Pat. No. 7,122,816, which isincorporated herein by reference in its entirety.

As mentioned above, the apparatus 227 includes first and second filters235, 240 through which the target mixture 230 is passed to removeimpurities such as the non-target particles from the target mixture 230.The first filter 235, which is optional, can be a sintered filter or amesh filter. The second filter 240 can be a filter that is anon-sintered, non-mesh filter that includes at least a set ofuniformly-sized through holes formed between opposing flat surfaces, asdescribed in greater detail below. The second filter 240 has a surfacethat is exposed to the target mixture 230, the exposed surface area ofsecond filter 240 can be at least a factor of one hundred less than anexposed surface area of a sintered filter that has an equivalenttransverse extent as the transverse extent of the second filter 240, asdescribed in greater detail when discussing FIGS. 18A and 18B. In someimplementations, the first filter 235 is also a non-sintered, non-meshfilter that also includes a set of uniformly-sized through holes formedbetween opposing flat surfaces.

The first filter 235 can be made from a first material and the secondfilter 240 can be made of a second material that is distinct from thefirst material. In this way, if the first material does not adequatelyremove the non-target particles from the target mixture 230 or if targetmaterial causes the first material to leach from the first filter intothe target mixture 230, then the second material can be selected to bedistinct from the first material to provide for the benefits notadequately provided for by the first material. Thus, the second materialcan be selected to remove the leached first material from the targetmixture 230 or to more adequately remove other non-target particles fromthe target mixture 230. For example, if the first material is titanium,then the second material can be tungsten or glass.

Moreover, the holes of the second filter 240 can have a cross-sectionalwidth that is different from a cross-sectional width of the holes of thefirst filter 235. Thus, in one implementation, the holes of the secondfilter 240 have a cross-sectional width that is less than thecross-sectional width of the holes of the first filter 235. In this way,the second filter 240 would be designed to remove smaller non-targetparticles in the target mixture 230. In other implementations, the holesof the second filter 240 have a cross-sectional width that is equal toor greater than a cross-sectional width of the holes of the first filter235. In this way, the second filter 240 can be designed to removenon-target particles that were introduced into the target mixture 230 bythe first filter 235.

Each hole of set of uniformly-sized through holes the second filter 240can have a cross-sectional width that is less than 10 μm in theimplementation in which the target material is tin. The cross-sectionalwidth of each of the through holes in the set of the second filter 240can be configured to vary no more than 20% from the cross-sectionalwidth of each of the other holes in the set of the second filter 240; inthis way, the second filter 240 can be said to have a set of“uniformly-sized” holes. Additionally, the cross-sectional width of eachof the holes of the second filter 240 is less than the cross-sectionalwidth of the orifice 255.

The width of a through hole is a distance that is measured along a crosssection that is in the transverse plane, which is the plane that isperpendicular to a longitudinal direction 241, which is labeled in FIG.2. The longitudinal direction 241 generally extends along the directiontraveled by the target mixture 230 as it travels from the second chamber205 toward the nozzle 250.

Referring to FIG. 3, another exemplary target material supply apparatus327 is designed similarly to the apparatus 227 in that it includes afirst chamber 300 and a second chamber 305 fluidly coupled to the firstchamber 300 by a pipe 310. What distinguishes the apparatus 327 from theapparatus 227 is that the apparatus 327 includes a first filter 335 thatis placed between the first chamber 300 and the second chamber 305 whilethe first filter 235 of the apparatus 227 is between the second chamber205 and the supply system 245.

Referring to FIG. 4, another exemplary target material supply apparatus427 is designed similarly to the apparatus 227 in that it includes afirst chamber 400 and a second chamber 405 fluidly coupled to the firstchamber 400 by a pipe 410. What distinguishes the apparatus 427 from theapparatus 227 is that the apparatus 427 lacks a first filter andincludes only a filter 440 that is placed between the second chamber 405and the supply system 445.

A description of an exemplary filter, which can be the second filter240, 340 of FIGS. 2 and 3, or the filter 440 of FIG. 4, or can be bothfilters 235, 240 and 335, 340 of FIGS. 2 and 3, is provided next withreference to FIGS. 5-14B along with a description of how the filter issecured within the target material supply apparatus. Referring first toFIG. 5, the filter 540 is arranged near an opening 565 of the secondchamber 505. The filter 540 is mounted between an outer face 567 of thesecond chamber 505 and a face 569 of a holder 571 that houses the supplysystem 545. The mount is such that the edges 573 of the filter 540 arehermetically sealed at the faces 567, 569 so that the target mixture 530flows through the through holes within the filter 540 and not around theedges of the filter 540. The edges 573 can be hermetically sealedbetween the faces 567, 569 using any suitable sealing system 575, suchas, for example, O-rings and/or metal gaskets.

Referring also to FIGS. 6A and 6B, the filter 540 is formed of a bulksheet-like substance in a solid phase that has a first flat surface 677that faces the second chamber (or reservoir 505) and a second flatsurface 679 that faces the orifice 555 of the nozzle 550. The materialof the bulk substance is selected to be compatible with the targetmixture to be filtered and the bulk substance can be a metal, metalalloy, or a non-metal.

The filter 540 includes a plurality of through holes 680 formed into thebulk substance and extending from the second flat surface 679. The holes680 are fluidly coupled at a first end 681 to the second chamber (orreservoir) 505 that holds the target mixture 530, and are fluidlycoupled at a second end 683 (which is at the second flat surface 679) tothe orifice 555 of the nozzle 550. In some implementations, all of theholes 680 are through holes such that the target mixture is able to passentirely through every one of the holes 680 of the filter 540.

At least a set of the through holes 680 of the filter 540 are uniformlysized in that their cross-sectional widths 685 do not vary by more thana maximum acceptable value from each other. Thus, for example, thecross-sectional width 685 of each through hole 680 of a set can vary nomore than 20% from the cross-sectional width 685 of each of the otherthrough holes 680 of the set. In this exemplary way, the filter 540 isformed with a design that is distinct from that of a sintered filter,which does not have uniformly-sized holes.

The number of through holes 680 and the cross-sectional width 685 of thethrough holes 680 in the filter 540 can be chosen based on theparticular target material to be passed through the filter 540, thenon-target particles to be blocked by the filter 540, or the necessarypressure drop maintained across the filter 540. For example, the numberof through holes 680 and the cross-sectional width 685 of each of thethrough holes 680 can be selected so that the pressure drop across thefilter 540 is negligible after the target mixture 530 fills a volume 547between the filter 540 and the nozzle 550 and the target mixture 530flows through the nozzle orifice 555. The cross-sectional widths 685 ofthe through holes 680 can be selected based on the size of thenon-target particles to be blocked so that the widths 685 are smallerthan the size of the non-target particles to be blocked. For example,the cross-sectional widths 685 can be less than about 10 μm.

Additionally, the holes 680 can form “channels” in that each throughhole 680 has a defined height 687 that extends from the second flatsurface 679 a long enough distance to form a channel. For example, insome implementations, the height 687 of a particular channel hole 680can be configured to be greater than or equal to the cross-sectionalwidth 685 of that through hole 680. In this way, the filter 540 isformed with a design that is distinct from that of a mesh because thefilter 540 has at least one flat surface 677, 679 and because each ofthe holes 680 of the filter 540 are formed as channels; with eachchannel having a defined height 687 that is at least 75% of thecross-sectional width 685 of the channel, at least 100% of thecross-sectional width 685 of the channel, or greater than 100% of thecross-sectional width 685 of the channel. For example, if thecross-sectional width 685 is 1 μm, then the height 687 can be greaterthan 1 μm and in one specific implementation the height 687 can be 10μm.

The filter 540, in this implementation, also includes a plurality ofopenings 690, each opening 690 extending between the first flat surface677 that faces the reservoir 505 and a group 691 of through holes 680such that the opening 690 fluidly couples the reservoir 505 to the group691 of through holes 680. Each opening has a cross-sectional width 693,and each through hole 680 in the group 691 has a cross-sectional width685 that is less than the cross-sectional width 693 of the opening 690to which that through hole 680 is fluidly coupled.

The through holes 680 can be cylindrically-shaped, that is, they canhave a cross-sectional shape that is cylindrical or approximatelycylindrical. The through holes 680 can have a uniform cross-sectionalwidth 685 along an axis 689 (which is perpendicular to the longitudinaldirection 241) of the through hole 680.

The through holes 680 can be formed using any suitable method. In FIGS.7-14B, an exemplary method is shown in which the through holes areetched into a bulk substance 743, and the steps are detailed below.

Referring to FIG. 7, the method starts with the bulk substance 743,which has a first flat surface 777 and a second opposing temporary flatsurface 769. The bulk substance 743 can be formed using any standardmachining or fabrication process. The bulk substance 743 can be made ofany suitable material that does not react adversely to the targetmaterial. Thus, if the target material is tin, then the bulk substance743 could be tungsten. Moreover, in one implementation, the width 753 ofthe bulk substance 743 can be between about 2 mm to about 10 mm and theheight 757 of the bulk substance 743 can be between about 10% to about50% of the width 753, which would be between about 0.2 mm to about 5 mmfor the exemplary height 757 noted above. The width 753 and the height757 of the bulk substance 743 are selected based on the application ofthe filter to be formed, the configuration of the nozzle, the reservoir,the ultimate size of the holes to be formed in the filter, and thepressure differential across the filter.

Referring to FIGS. 8A and 8B, a plurality of openings 790 are formed inthe bulk substance 743, the openings 790 extending from the firstsurface 777 to the temporary second surface 769. The openings can beformed as an array (a regular pattern as shown in FIG. 8A) of openingsor they can be randomly placed throughout the substance 743. In someimplementations, the openings 790 are formed using standard machiningprocesses such as milling or drilling. In other implementations in whichthe substance 743 is a substance that is too hard to machine usingtraditional techniques and is electrically conductive, then the openings790 can be formed using electrical discharge machining (EDM). In otherimplementations, the openings 790 are formed using lithography oretching techniques.

The number of openings 790 that are formed and the width 793 of eachopening 790 are determined at least in part by the pressure limits ofthe bulk substance 743 and by the desired number of holes that areneeded for filtering. In an implementation in which the bulk substance743 is tungsten, the width 793 of an opening 790 can range between about20 μm to about 500 μm.

Referring next to FIG. 9, the openings 790 are each filled with a fillersubstance 993 that exhibits adequate differential etching relative tothe bulk substance 743. The filler substance 993 can be any etchablepolymer or other substance as used in lithographic applications if thebulk substance 743 is tungsten. Referring to FIG. 10, the fillersubstance 993 is polished to be flush with the second temporary surface769. Next, as shown in FIG. 11, a layer 1143 of the material that makesup the bulk substance 743 is deposited on the second temporary surface769. Thus, as an example, the layer 1143 is made of tungsten if the bulksubstance 743 is made of tungsten. In the example in which the bulksubstance743 has a height 757 of about 1 mm and a width 753 of about 5mm, the thickness 1157 of the layer 1143 can be between about 5 μm andabout 15 μm.

Next, as shown in FIG. 12, the filler substance 993 is removed by, forexample, etching to form the openings 690. Then, as shown in FIGS. 13Aand 13B, a photoresist 1395 is applied to the layer 1143 and thephotoresist 1395 is patterned with holes 1397. Referring to FIGS. 14Aand 14B, the bulk substance 743 is etched to transfer the photoresistpatterned holes 1397 into the bulk substance 743 to form the throughholes 680. After the through holes 680 are formed, then the photoresist1395 is removed to form the completed filter, as shown in FIGS. 6A and6B.

Referring to FIG. 15, in another implementation, a filter 1540 can beformed as a capillary array. The filter 1540 is arranged near an opening1565 of the second chamber 505 and is mounted between an outer face 1567of the second chamber 505 and a face 1569 of a holder 1571 that housesthe supply system 545. The mount is configured such that the edges 1573of the filter 1540 are hermetically sealed at the faces 1567, 1569 sothat the target mixture 530 flows through the through holes within thefilter 1540 and not around the edges of the filter 1540. The edges 1573can be hermetically sealed between the faces 1567, 1569 using anysuitable sealing system 1575, such as, for example, O-rings and/orgaskets.

Referring also to FIGS. 16 and 17, the filter 1540 is formed of a bulksubstance 1643 in a solid phase that has a first flat surface 1677 thatfaces the second chamber (or reservoir 505) and a second flat surface1679 that faces the orifice 555 of the nozzle 550. The material of thebulk substance 1643 is selected to be able to withstand high pressuredifferentials. In some implementations, the bulk substance 1643 is madeof titanium. The bulk substance 1643 is formed with a tapered passage1669 that has a geometry that accommodates a collimated hole structure1680, which is made of any material that is compatible with the targetmixture 530 to be filtered. In some implementations, the structure 1680is made of a non-metal such as glass.

The collimated hole structure 1680 includes a cladding 1693 and an innerregion 1697 that fits within the tapered passage 1669, and the throughholes are formed in the inner region 1697. The through holes in theregion 1697 can have a cross-sectional width that is comparable to thecross-sectional width 685 of each through hole 680 of the filter 540.For example, the cross-sectional width of each through hole in the innerregion 1697 can be between about 0.5 μm to about 2.0 μm for a targetmaterial such as tin. Additionally, the height of each through hole inthe inner region 1697 can be between about 1-10 mm. The region 1697 caninclude at least 10,000 through holes.

The through holes formed into the region 1697 extend between the secondflat surface 1679 and the first flat surface 1677, and they are fluidlycoupled at a first end 1681 to the second chamber (or reservoir) 505that holds the target mixture 530, and are fluidly coupled at a secondend 1683 to the orifice 555 of the nozzle 550.

At least a set of the through holes of the filter 1540 can be uniformlysized in that their cross-sectional widths do not vary by more than amaximum acceptable value from each other. Thus, for example, thecross-sectional width of each through hole of the set can vary no morethan 20% from the cross-sectional width of each of the other throughholes of the set. In this way, the filter 1540 is formed with a designthat is distinct from that of a sintered filter, which does not have aset of uniformly-sized holes.

Additionally, the number of through holes and the cross-sectional widthof each of the through holes in the filter 1540 can be chosen based onthe particular target material to be passed through the filter 1540, thenon-target particles to be blocked by the filter 1540, or the necessarypressure drop maintained across the filter 1540. For example, the numberof through holes and the cross-sectional width of the through holes canbe selected so that the pressure drop across the filter 1540 isnegligible after the target mixture 530 fills a volume 1547 between thefilter 1540 and the nozzle 550 and the target mixture 530 flows throughthe nozzle orifice 555. The cross-sectional widths of the through holescan be selected based on the size of the non-target particles to beblocked so that the widths are smaller than the size of the non-targetparticles to be blocked.

Additionally, as discussed above with respect to the filter 540, thethrough holes of the filter 1540 can form “channels” in that eachchannel hole has a defined height that extends from the second end 1683a long enough distance to form a channel. For example, the height of aparticular channel hole can be configured to be greater than or equal tothe cross-sectional width of that channel hole. In this way, the filter1540 is formed with a design that is distinct from that of a meshbecause the filter 1540 has at least one flat surface 1677, 1679 andbecause each of the holes of the filter 1540 are formed as channels;with each channel having a defined height that is at least 75% of thecross-sectional width of the channel, at least 100% of thecross-sectional width of the channel, or greater than 100% of thecross-sectional width of the channel.

The through holes of the filter 1540 can be cylindrically-shaped, thatis, they can have a cross-sectional shape that is cylindrical orapproximately cylindrical. Moreover, the through holes can have auniform cross-sectional width along an axis 1689 of the through hole.

The through holes in the region 1697 of the filter 1540 are formed usinga glass drawing process, an etching process, or using drawn tubestructures, as detailed by Collimated Holes, Inc. of Campbell, CA(http://www.collimatedholes.com/products.html).

Referring also to FIGS. 18A and 18B, a filter 1840 is shown next to asintered filter 1898 having an equivalent transverse extent TE to thatof the filter 184. The filter 1840 is a generalized representation ofthe filters described above, and it is not drawn to scale in FIG. 18A,but is shown merely to provide context for the following description.The transverse extent TE is a size of the filter 1840 or the sinteredfilter 1898 that extends along a direction that is transverse to thelongitudinal direction 241 defined by the flow of the target mixturethrough the filter 1840. The filter 1840 has a surface 1877 that extendsalong a plane that is transverse to the longitudinal direction 241 andthe sintered filter 1898 has a surface 1899 that extends along a planethat is transverse to the longitudinal direction 241. The surface 1877is the surface near the through holes that remove at least some of thenon-target particles of the target mixture that impinges upon the filter1840; thus, the target mixture contacts the surface 1877.

When used in the target material supply apparatus, the filter 1840 has asurface area that is exposed to the target mixture and this is referredto as the exposed surface area of the filter 1840; and the sinteredfilter 1898 has a surface area that is exposed to the target mixture andthis is referred to as the exposed surface area of the filter 1898. Inthe situation in which the filter 1840 and the sintered filter 1898 havethe same transverse extent, the exposed surface area of the filter 1840is significantly less than an exposed surface area of the sinteredfilter 1898. Thus, for example, if the filter 1840 and the sinteredfilter 1898 have the same transverse extent, the exposed surface area ofthe filter 1840 is less than one hundredth of the exposed surface areaof the sintered filter 1898; the exposed surface area of the filter 1840is less than one ten thousandth of the exposed surface area of thesintered filter 1898; or the exposed surface area of the filter 1840 isless than one millionth of the exposed surface area of the sinteredfilter 1898.

Because of this significant reduction in exposed surface area, thefilter 1840 is less likely to clog and more easily transfers the targetmaterial through while blocking the non-target particles than thesintered filter 1898 of the same transverse extent.

Other implementations are within the scope of the following claims.

For example, the target material supply apparatus 127 can have only onechamber or more than two chambers. The filters can be placed after anyof the chambers of the apparatus 127, depending on the situation, aslong as the filter with the uniformly-sized through holes and/or thelower surface area is placed between a reservoir of the target mixtureand the nozzle.

For example, the filter 240, 340, 440 can be formed by micromachining anarray of channel holes to create a sieve, by assembling a bundle offibers of material (such as ceramic) compatible with the targetmaterial, by placing a filter media (such as quartz) in a larger sectionof quartz that is fused to a nozzle, or with a quartz tube having anin-situ frit filter (for example, chromatography capillary).

What is claimed is:
 1. An apparatus comprising: a supply systemcomprising an orifice; a holder that houses the supply system, theholder comprising a face that opens to the orifice; and a filtercomprising a plurality of channels extending from a first channel end ata first surface of the filter to a second channel end at a second,opposing surface of the filter, the first channel end of each channelbeing configured to be fluidly coupled at a first end to a reservoir,and the second channel end of each channel being configured to befluidly coupled to the orifice, each channel comprising: a plurality ofthrough holes extending from the second surface of the filter; and anopening extending from the first surface of the filter toward theplurality of through holes, the opening being between the first channelend and the plurality of through holes such that the opening fluidlycouples the reservoir to the through holes; wherein the opening has across-sectional width, and each through hole has a cross-sectional widththat is smaller than the width of the opening, the through holes aresized to remove at least some non-target particles from a targetmixture, the through holes are within the cross-sectional width of theopening, and the filter is hermetically sealed to the face of theholder, the channel has a width no larger than the cross-sectional widthof the opening, and the plurality of through holes and the opening arefluidly coupled such that substantially all fluid that flows into thefirst end of the channel flows in the channel.
 2. The apparatus of claim1, wherein the through holes have a uniform cross-sectional width alongan axial length of the through hole.
 3. The apparatus of claim 1,wherein the through holes are a capillary array.
 4. The apparatus ofclaim 1, further comprising: a radiation source that supplies radiationto a target location; and wherein the supply system supplies the targetmixture to the target location such that a plasma is formed when atarget material within the target mixture is irradiated by the suppliedradiation.
 5. The apparatus of claim 2, wherein the through holes andthe opening are cylindrically shaped.
 6. The apparatus of claim 1,wherein the filter comprises tungsten.
 7. The apparatus of claim 1,wherein the filter comprises silicon nitride.
 8. The apparatus of claim1, wherein the filter comprises a bulk structure, and the first andsecond surfaces of the filter are flat.
 9. The apparatus of claim 8,wherein the openings are arranged in a regular pattern on the first flatsurface.
 10. A filter for use in a target material supply apparatus, thefilter comprising: a sheet having a first flat surface and a secondopposing flat surface, the first flat surface being in fluidcommunication with a reservoir that holds a target mixture that includesa target material and non-target particles; a plurality of groups ofthrough holes extending from the second flat surface and being fluidlycoupled at the second flat surface to an orifice of a nozzle; and aplurality of openings extending from the first flat surface, eachopening fluidly coupling a group of through holes to the reservoir suchthat one opening couples one group of through holes to the reservoir,wherein the sheet has a surface area that is exposed to the targetmixture, the through holes have a uniform cross sectional width along anaxial length of the through hole, each of the through holes is part ofone group of through holes, each group of through holes is within across-sectional width of the opening that couples the group of throughholes to the reservoir, the through holes are arranged in a regularpattern on the second flat surface, and the sheet comprises a singlepiece of a solid phase bulk material.
 11. The filter of claim 10,wherein a cross-sectional width of each through hole of the plurality ofthrough holes varies no more than 20% from the cross-sectional width ofeach of the other through holes of the plurality of through holes andthe cross-sectional width of each hole of the plurality of through holesis less than a height of the hole.
 12. The filter of claim 10, whereineach through hole has a height along a direction that is parallel to alongitudinal axis of the through hole and a width in a direction that isperpendicular to the direction, the height being at least 75% of thewidth.
 13. The filter of claim 10, wherein the openings have a uniformcross sectional width along an axial length of the opening.
 14. Theapparatus of claim 10, wherein the filter comprises tungsten.
 15. Theapparatus of claim 10, wherein the filter comprises silicon nitride. 16.The apparatus of claim 1, wherein the through holes are in direct fluidcommunication with the opening.
 17. The apparatus of claim 1, wherein across-sectional width of each through hole of the plurality of throughholes varies no more than 20% from the cross-sectional width of each ofthe other through holes of the plurality of through holes and thecross-sectional width of each hole of the plurality of through holes isless than a height of the hole.
 18. The apparatus of claim 1, whereinthe filter is hermetically sealed to the face of the holder with one ormore of an O-ring and a gasket.
 19. The apparatus of claim 1, whereinthe filter defines an edge, and the edge of the filter is hermeticallysealed to the face of the holder.
 20. A filter for use in a targetmaterial supply apparatus of an extreme ultraviolet (EUV) light source,the filter comprising: a bulk material comprising a plurality ofchannels extending from a first channel end at a first surface of thebulk material to a second channel end at a second surface of the bulkmaterial, the first channel end of each channel being configured to befluidly coupled at a first end to a reservoir, and the second channelend of each channel being configured to be fluidly coupled to anorifice, each channel comprising: a plurality of through holes extendingfrom the second surface of the filter; and an opening extending from thefirst surface of the filter toward the plurality of through holes, theopening being between the first channel end and the plurality of throughholes such that the opening fluidly couples the reservoir to the throughholes; wherein the opening has a cross-sectional width, and each throughhole has a cross-sectional width that is smaller than the width of theopening, the through holes are sized to remove at least some non-targetparticles from a target mixture, the through holes are within thecross-sectional width of the opening, and the channel is configured tocontain a fluid that flows into the first end of the channel in achannel width, the channel width being no larger than thecross-sectional width of the opening.
 21. The filter of claim 20,wherein the bulk material is a single, non-layered piece of a solidphase bulk material.
 22. The filter of claim 21, wherein the bulkmaterial comprises tungsten.
 23. The filter of claim 21, wherein thefilter comprises silicon nitride.
 24. The filter of claim 20, whereinthe bulk material comprises tungsten.
 25. The filter of claim 20,wherein the filter comprises silicon nitride.
 26. The filter of claim20, wherein each through hole has a height along a direction that isparallel to a longitudinal axis of the through hole and a width in adirection that is perpendicular to the direction, the height being atleast 75% of the width.
 27. The filter of claim 20, wherein the openinghas a uniform cross sectional width along an axial length of theopening.